This invention relates to a circuit arrangement for supplying a lamp, comprising a DC/AC converter for generating, from an input voltage, a high-frequency lamp current at a frequency f, and provided with
input terminals for connection to terminals of a power supply source supplying the input voltage, PA1 a first branch interconnecting the input terminals and comprising a series arrangement of a first switching element and a second switching element, PA1 a first control circuit coupled to a control electrode of the first switching element for rendering the first switching element conducting and non-conducting, PA1 a second control circuit coupled to a control electrode of the second switching element and also coupled to a point P of the first control circuit for rendering the second switching element conducting dependent upon the voltage present at the point P, and PA1 a load circuit shunting one of the switching elements and having terminals for connection of a lamp.
A circuit arrangement of this type is known from EP 0 294 878. In the known circuit arrangement, the first control circuit comprises a series arrangement of an inductive element L1 and a capacitive element C1, which series arrangement shunts a part of a ballast coil arranged in series with the lamp. The point P is constituted by a common point of the inductive element L1 and the capacitive element C1, and the second control circuit is connected to the point P via a diode. One end of the capacitive element C1 remote from the point P is connected to a common point A of the first and the second switching element of the first branch. Since the ballast coil conveys the lamp current during operation, alternating voltages of the frequency f are present both across the inductive element L1 and across the capacitive element C1. The AC voltage across the capacitive element C1 is used as a control signal for rendering the first switching element conducting and non-conducting. The control of the second switching element is derived from the control of the first switching element, i.e. the second switching element is rendered conducting when the voltage at the point P has reached a relatively low value. During normal operation of the circuit arrangement, this value is reached when the first switching element has been rendered non-conducting. The value of the voltage at the point P is determined by the voltage across the capacitive element C1 and the voltage at the point A. If the second control circuit is to function properly, it is necessary that the voltage at point A, after the first switching element has become non-conducting, decreases to a value which is equal to the voltage at the negative terminal of the power supply source, or is smaller than this voltage. However, practice has proved that, for example, in the case of a relatively low amplitude of the input voltage or in the case of the lamp having rectifying properties, the voltage at point A decreases to an insufficient extent, so that the second switching element is not rendered conducting and the DC/AC converter stops functioning.